Orthogonal Frequency Division Multiplexing (OFDM) is included in various terrestrial standards, for example, long-term evolution (LTE) fourth generation (4G), and in certain wireless local area network (WLAN) protocols. OFDM generally requires the OFDM receiver to accurately and stably synchronize to OFDM signals that arrive at the receiver antenna(s). Synchronization error can induce loss of orthogonality among the OFDM subcarriers, resulting in degradation beyond what would be experienced by traditional systems. In addition, satellite systems can employ powerful low density parity check (LDPC) coding, requiring receivers to operate at low levels of signal-to-noise ratio (SNR), which can further complicate the synchronization task.
One synchronization technique, commonly referred to as the “Schmidl and Cox” technique (See, T. M. Schmidl and D. C. Cox, “Robust Frequency and Timing Synchronization for OFDM,” IEEE Trans. on Communications, Vol. 45, pp. 1613-21, December 1997), provides timing and frequency synchronization by sending two OFDM training symbols, the first containing identical halves. This technique, however, has technical shortcomings. For example, the second OFDM training symbol provides for estimation of only the even integer part of the frequency offset. This limits the accuracy of offset estimation that can be obtained from the training symbols, and also necessitates additional computation. Another technical shortcoming of the Schmidl and Cox is a requirement of high SNR, due to its differential operation.
Another technique for OFDM timing and frequency synchronization, is a closed-loop method using a feedback loop that includes a post Fast Fourier Transform (FFT) estimation. The closed loop, though, can induce instability in some applications.